In this work, a-C:H:Si (DLC-Si) films were produced onto polycarbonate substrates by the rf-PACVD technique from gaseous mixtures of CH4 + SiH4 and C2H2 + SiH4. The effects of self-bias and gas composition upon mechanical and optical properties of the films were investigated. Films deposited at low self-bias voltages showed large optical gaps, decreasing for increasing bias. Addition of silane to the discharge gas was observed to further increase the optical gap. High scratch hardness values were obtained at intermediate values of self-bias for both the films deposited from CH4 + SiH4 and C2H2 + SiH4. The addition of silane induced a minor decrease in scratch hardness values. No direct correlation of scratch hardness was found with the microhardness or residual stress of the coatings. (C) 2003 Elsevier Science B.V. All rights reserved.